It is well known to use a tachometer system to determine the speed of rotation of the wheels of the landing gear of an aircraft. Landing gear incorporating a known tachometer system is shown in FIG. 1. The landing gear 1 comprises a wheel 2. The landing gear 1 comprises a motor 3 for the brake cooling fans 8 of the landing gear 1, a shroud 7 to cover the brake cooling fans and a debris guard 9 to prevent large debris from getting into the fan mechanism. A tachometer 5 is mounted upon a tachometer shaft 6. As the wheel 2 rotates, the debris guard 9 rotates (as it is coupled to the wheel 2), and in turn the tachometer shaft 6 rotates (as it is coupled to the debris guard 9). As the tachometer shaft 6 rotates, it rotates the rotor of the tachometer 5. The stator of the tachometer 5 remains stationary, as it is couples to the motor 3, which is coupled to the axle of the wheel 2, which remains stationary during rotation of the wheel 2. In this way, the interaction of the rotor and stator of the tachometer 5 during rotation of the wheel 2 cause the tachometer 5 to generate a variable voltage electrical signal.
This variable voltage signal is processed to produce a speed value, as shown in FIGS. 2 and 3. FIG. 2 is a schematic diagram of the known tachometer system 11.
The variable voltage signal of the tachometer 5 is a raw alternative current (AC) signal 30, which the voltage of which varies as a sine wave as the wheel 2 rotates. The frequency and voltage of the raw signal 30 are proportional to the rotation speed of the wheel 2. The raw signal 30 is fed to a Schmitt trigger 20, which uses upper and lower thresholds THR_UP and THR_DOWN respectively to convert the raw signal 30 into a stepped signal 31, by outputting a “high” value when the raw signal 30 moves above the upper threshold THR_UP, and a “low” value when the raw signal 30 moves below the lower threshold THR_DOWN. As well as converting the raw signal 30 to a signal that can be more easily processed, by setting the thresholds THR_UP and THR_DOWN at sufficiently high magnitudes (for example +1.5V and −1.5V respectively), unwanted noise in the raw signal 30 is eliminated.
The stepped signal 31 is then fed to a processer 21, which uses embedded software to perform further processing of the signal. First, a “tops” signal 32 is generated by outputting a “tick” (i.e. temporarily outputting a high value and otherwise outputting a low value) when the stepped signal 31 transitions from its high value to its low value. A counter signal 33 is then generated from the tops signal 32, in which the counter signal 33 is increased in proportion to the length of time since the last tick of the tops signal 32. In other words, the counter signal 33 is a timer that is reset by a tick of the tops signal 32, that counts the time between the ticks of the tops signal 32. A speed signal 34 is then generated from the counter signal 33, in which the level of speed signal 34 is the level of the counter signal 33 before it was last reset to zero, i.e. just before the previous tick of the tops signal 32. (So as shown in FIG. 3, the speed signal 34 is set at level T1 when the counter signal 33 drops from T1 to zero, and is then set at level T2 when the counter signal 33 drops from T2 to zero, and so on.) The speed signal 34 indicates the determined speed of the wheel 2, and is the output of the processor 21.
While known tachometer systems used for aircraft are designed to be accurate at high speeds, for example at speeds relevant to anti-skid systems, they are generally not accurate at low speeds. This is because at low speeds the raw signal 30 output by the tachometer 5 has low signal-to-noise ratio a low voltage, and becomes dominated by electrical noise and mechanical noise.
It would be advantageous to provide a tachometer system for landing gear that was accurate at low speeds as well as high speeds. This is particularly the case where the landing gear incorporates an electrical drive system, as synchronisation of the speed of the drive motor with speed of the wheel is required to avoid shock loading when then electrical drive system is engaged with the wheel, and also for other low-speed applications such as providing an indication of whether an aircraft is moving or stationary. It would be particularly advantageous to achieve this using existing tachometers, despite their low signal-to-noise ratios at low speeds, so special and/or additional tachometers are not required. Further, known tachometer systems such as the tachometer system 1 usually cannot be used at low speeds at all, as their filtering systems will not accept voltages at the low voltage magnitudes output by known tachometers at low speeds (for example below +/−0.4V), due to the low signal-to-noise ratio.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved tachometer systems and improved methods of determining the rotation speed of a wheel of a landing gear of an aircraft.